1. Field
The present disclosure generally relates to optical devices. More specifically, the present disclosure relates to an optical multiplexer/de-multiplexer (MUX/de-MUX) that includes a compact echelle grating and a two-dimensional, sub-wavelength pattern of features in a propagation region, which can make the optical MUX/de-MUX compatible with CMOS fabrication.
2. Related Art
Integrated silicon photonics is a promising new technology that provides a number of advantages for chip-level communication, such as very high index contrast and compatibility with CMOS fabrication technology. Ongoing research into integrated silicon photonics is focusing on opportunities to provide: low latency, high bandwidth, high density, and low power consumption. To date, several key active elements, including silicon lasers, modulators, and photodetectors, have been realized in silicon using low-cost CMOS-compatible processes. However, an ideal wavelength filter (such as an optical multiplexer/de-multiplexer or MUX/de-MUX), which can be used in wavelength division multiplexing (WDM), has not been developed yet.
In a high-data-rate WDM application, an ideal wavelength filter has: low loss, low crosstalk, a flat passband, accurate wavelength-channel alignment with minimal wavelength tuning, and a small footprint. In principle, a number of optical MUX/de-MUX designs can be integrated with CMOS circuits, including: arrayed waveguide gratings (AWGs), ring resonators, and planar concave gratings (echelle gratings). AWGs on silicon dioxide offer stability, low loss, low crosstalk, and do not require tuning. However, AWGs on silicon are typically sensitive to fabrication errors and temperature variations. In contrast, ring resonator-based wavelength filters can be very compact (for example, approximately 0.001 mm2 per channel), and they can be fabricated using a CMOS-compatible process. However, ring resonators are often sensitive to fabrication errors and temperature variations, and thus typically require active tuning. In addition, a single ring can usually independently add or drop only one wavelength channel. As a consequence, a bank of ring resonators may be needed for a multi-channel WDM link.
Echelle gratings, which image and diffract optical signals, are particularly interesting because they can be designed to have: low optical crosstalk, fixed channel spacing, reduced tuning and monitoring requirements, and low on-chip optical loss. In addition, an echelle grating can simultaneously multiplex or demultiplex multiple channels; for example, up to 128 wavelength channels are possible.
However, echelle grating designs with high optical performance (such as low optical crosstalk and image aberrations) often have larger footprints. For example, image aberrations associated with grating facets far from the center of an echelle grating are one of the main contributors to optical crosstalk. As a consequence, wider entrance and exit apertures along the Rowland circle are usually required in order to reduce the input beam divergence in the free-propagation region of an echelle grating. In addition, smaller diffraction angles (less than 40°) are typically used to avoid illuminating grating facets further away from the center of the echelle grating. Given linear dispersion, these design choices result in a significantly longer focal length for the echelle grating; thus, the device size or footprint is significantly increased.
Increasing the size of echelle gratings usually increases the manufacturing cost. In addition, it is often difficult to fabricate larger echelle gratings using CMOS-compatible processes because the large bulk un-etched silicon slab in the echelle gratings violates CMOS design rules. In particular, by degrading the quality of the chemical-mechanical-polishing (CMP) process for the whole wafer, large echelle gratings are a significant yield risk to other devices on the wafer and, thus, are prohibited. This incompatibility with CMOS processes significantly increases the cost and complexity of using echelle gratings in integrated silicon photonics.
Hence, what is needed is an optical MUX/de-MUX that does not suffer from the above-described problems.